Among the key properties that distinguish adult mammalian stem cells from their more differentiated progeny is the ability of stem cells to remain in a quiescent state for prolonged periods of time (Li & Clevers, Science 327, 542-545 (2010)), (Fuchs, Cell 137, 811-819 (2009)). Quiescence is a common feature of stem cells characterized by reversible mitotic arrest and reduced metabolic activity that protects stem cells against endogenous stress caused by DNA replication and cellular respiration. Quiescence of stem cells is critical to ensure lifelong tissue maintenance and to protect the stem cell pool from premature exhaustion under conditions of various stresses. However, the molecular pathways for the maintenance of stem-cell quiescence remain elusive.
Stem cells have a capacity both for self-renewal and the generation of differentiated cell types, which provides the possibility for therapeutic regeneration of cells and tissues in the body. In addition to studying the important normal function of stem cells in the regeneration of tissues, researchers have further sought to exploit the potential of in situ and/or exogenous stem cells for the treatment of a variety of disorders. While early, embryonic stem cells have generated considerable interest, the stem cells resident in adult tissues also provide an important source of regenerative capacity.
These somatic, or adult, stem cells are undifferentiated cells that reside in differentiated tissues, and have the properties of both self-renewal and generation of differentiated cell types. The differentiated cell types may include all or some of the specialized cells in the tissue. For example, hematopoietic stem cells give rise to all hematopoietic lineages, but do not seem to give rise to stromal and other cells found in the bone marrow. Sources of somatic stem cells include bone marrow, blood, the cornea and the retina of the eye, brain, skeletal muscle, cartilage, bones, dental pulp, liver, skin, the lining of the gastrointestinal tract, and pancreas, and the like. Adult stem cells are usually quite sparse. Often they are difficult to identify, isolate, and purify. Often, somatic stem cells are quiescent until stimulated by the appropriate growth signals.
Muscle tissue in adult vertebrates regenerates from stem cells known as satellite cells. Satellite cells are distributed throughout muscle tissue and are mitotically quiescent in the absence of injury or disease, residing in an instructive, anatomically defined niche. The satellite cell niche constitutes a distinct membrane-enclosed compartment within the muscle fiber, containing a diversity of biochemical and biophysical signals that influence satellite cell function. In addition to satellite cells, cell types that contribute to muscle regeneration include mesangioblasts, bone marrow derived cells, muscle interstitial cells, mesenchymal stem cells, etc. See D. D. Cornelison et al. (2001) Dev Biol 239, 79; S. Fukada et al. (2004) Exp Cell Res 296, 245; D. Montarras et al. (2005) Science 309, 2064; S. Kuang et al. (2007) Cell 129, 999; M. Cerletti et al. (2008) Cell 134, 37; C. A. Collins et al. (2005) Cell 122, 289; A. Sacco et al. (2008) Nature 456, 502; R. I. Sherwood et al. (2004) Cell 119, 543; Sampaolesi et al. (2003) Science 301(5632):487-92; and Galvez et al. (2006) J Cell Biol. 174(2):231-43.
Satellite cells are the primary cells in muscle tissue required for the regeneration that occurs in response to injury or disease. In response to injury, SCs are activated and they proliferate and differentiate into myoblasts that undergo further differentiation and fusion to form muscle fibers.
One of the current limitations for stem cell therapeutics is the inability to manipulate stem cells in vitro after isolation without the loss of their potency. Emerging data suggest that stem cells potency depends on their capacity to remain in a quiescent state prior their activation induced by regenerative stimuli such as injury. For many stem cell populations, such as skeletal muscle stem cells (MuSCs), hematopoietic stem cells (HSCs), or neural stem cells (NSCs), the most potent cell in terms of transplantation efficacy and their ability to repair and repopulate the tissue is the long-term quiescent stem cell. It has been estimated that such cells can remain in the quiescent state for months in mice and years in humans. Physiologically, stem cells reside within the tissue in a specialized microenvironment or “niche”, which is characterized by a unique combination of biophysical, biochemical and cellular properties that promote stem cell quiescence in several tissue compartments. Among the major hurdles, in the basic studies and translational applications of stem cell biology, is the ability to mimic the endogenous niche ex vivo.
To date, bioengineered niches have focused almost exclusively on those aspects that influence the dynamics of dividing cells, allowing studies of cell replication and cell fate determination. What has not been well modeled is the endogenous niche that promotes and maintains stem cell quiescence. The major challenge to create such culture conditions is the fact that as soon as quiescent cells are isolated from their in vivo niche and plated in conventional culture conditions, they immediately begin exiting quiescence, activating, and undergoing the processes of proliferation and differentiation. The ability to maintain stem cells in a quiescent state ex vivo would be a major advance for the study of the biology of the quiescent state. Even more importantly, from a translational perspective, the isolation and culturing of quiescent stem cells followed by transplantation should preserve cell potency.
To be able to maintain stem cells in a potent, quiescent state ex vivo while allowing, for example, genetic manipulation prior to transplantation, would greatly enhance their therapeutic potential. Specifically, given the high potency of these cells, this approach would abrogate the need for expansion of progenitors in vitro and focus attention instead on the ability of very few quiescent stem cells to replace vast amounts of tissues, as has been shown for MuSCs, HSCs and NSCs. MuSCs, or “satellite cells”, reside in a quiescent state under the basal laminae of muscle fibers. The transplantation potency of freshly isolated adult MuSCs is rapidly lost when they undergo activation or proliferation in vitro.